1. Field of the Invention
The present invention relates to a current collector which is attached to a movable body such as electric rolling stock, an overhead crane or the like, and which receives current from a feed.
2. Description of the Related Art
A typical current collector attached to a movable body is generally constructed such that a current collector shoe assembly is attached to a top end of a frame which is mounted at its base end on the movable body, and that such current collector shoe assembly is pressed to a feeder by a main spring applying extensible force to the frame. When the movable body starts moving in this condition, such a pressing force of the current collector shoe assembly varies influenced by vibration of the feeder and a vehicle, and the uneveness of the feeder, or the like. In the worst case, the pressing force might be reduced to nothing, thereby removing the current collector shoe assembly from the feeder, that is, causing pantograph bounce. This further lowers the vehicle performance and causes wear on the feeder and the current collector shoe assembly.
Accordingly, in order to prevent such pantograph bounce, the conventional 1 degree of freedom system including only a main spring is increased by degrees up to 2, 3, and even greater, thereby aiming at improving the performance of the current collector shoe assembly. The relationships between the vibration frequency f of the 1 degree of freedom system and the 2 degrees of freedom system and the allowable amplitude A are shown in FIGS. 10A, 10B and 10C.
In the 1 degree of freedom system of FIG. 10A, S1 is a curve indicating a solution of a vibration equation, and in the range R1 lower than the characteristic curve S1, the vibration of the current collector shoe assembly is allowed, thus enabling normal current collecting without pantograph bounce. In FIG. 10B, S2 and S3 represent the curves indicating the two solutions of the vibration equation of the 2 degrees of freedom system, and in the range R2 having the smaller amplitude A than the smaller value of the two characteristic curves S2 and S3, current collecting can be performed normally.
As will be seen from FIG. 10C as a result of overlapping FIGS. 10A and 10B, the characteristics of the 1 degree of freedom system are superior to those of the 2 degrees of freedom system in the range R3 of a relatively low frequency, and vice versa in the range R4 of a relatively high frequency. That is, the 1 degree of freedom system indicated by the characteristic curve S1 has low current collecting performance (high ratio of pantograph bounce) in a high frequency range, but when the degree of freedom increases, the current collecting performance can be improved in a high frequency range. However, as will be clearly seen from FIG. 10B, in a multi-degrees of freedom system having 2 or greater degrees, the current collecting performance is sharply reduced around the intrinsic frequency f.sub.0 of the system. Hence, the closeness of the values of intrinsic frequency of the current collector and that of the movable body causes the continuous occurrence of pantograph bounce, thus considerably reducing the current collecting performance.
Further, in order to prevent pantograph bounce of the current collector of the 1 degree of freedom system, a technique is known such that a single damper applies a damping force only to the downward movement (compressing dimension) of the frame (Masayuki ONODERA "The Latest Techniques and Construction of the Pantograph", Science of Electric Rolling Stock, Vol. 45, No. 3, (March 1992), issued by Electric Rolling Stock Society, pp. 37-39). According to this technique, the pantograph bounce of the shoe assembly due to great vibration force from the feeder can be inhibited to some extent, but the current collecting performance cannot be improved in a high frequency range.